A magnetoresistive random access memory (MRAM) which is an integrated magnetic memory utilizing a tunnel magnetoresistive effect (TMR) technology has been recently attracting attention. TMR elements used in the MRAM include an in-plane magnetization type element described in Non Patent Document 1 and a perpendicular magnetization type element described in Non Patent Document 2. In the in-plane magnetization type, the magnetization directions of a free layer and a reference layer form magnetic rotation in a direction perpendicular to a stacking direction of stacked layers forming the element. In the perpendicular magnetization type, the magnetization directions of the free layer and the reference layer form magnetic rotation in the same direction as the stacking direction of the stacked layers forming the element.
In manufacturing of the TMR elements, there is widely used a sputtering method (hereafter, also simply referred to as sputter) of sputtering a target made of a desired film formation material to form a film on a substrate opposed to the target. As a method of forming an insulating film used as a tunnel barrier layer in a TMR element, there is known a method of performing RF sputtering of an oxide target to form an oxide film (see Patent Document 1). Moreover, a method of forming an oxide film by forming a metal film, followed by oxidation treatment (see Patent Document 2) is also known.